(i) Field of the Invention
This invention relates to an integral process for, and to an independent, procedure leading to, the production of an aqueous chlorine dioxide solution containing virtually no chlorine.
(ii) Description of the Prior Art
Various processes which have been proposed heretofore for producing chlorine dioxide yield chlorine dioxide contaminated with chlorine formed as a by-product.
One of such methods for generating chlorine dioxide is to reduce a chlorate with a reducing agent in a strong acid. The reactions which occur are exemplified below, wherein for the sake of illustration, the chlorate used is sodium chlorate and the reducing agent is hydrochloric acid. EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O(1) EQU NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O (2)
Reaction (1) is the desired reaction, while reaction (2) is undesired and produces chlorine only. Catalysts might be found either to speed up reaction (1) or to inhibit reaction (2). However, even if reaction (2) does not proceed, the purity of the chlorine dioxide produced is only ##EQU1##
In order to purify the chlorine dioxide product to a high degree, the chlorine gas accompanying the chlorine dioxide must be almost completely eliminated from the resulting material. The substantial elimination of the chlorine gas therefrom is very difficult, resulting in the process being costly.
As disclosed by U.S. Pat. No. 2,641,528 issued Jun. 9, 1953 to Max L. Audoynaud, one prior method for producing chlorine dioxide relatively free from chlorine involved changing the basic chemistry, by passing an inert gas into a hot solution of a metallic chlorate in concentrated sulfuric or phosphoric acid. Under these conditions, it was claimed, a gaseous mixture rich in chlorine dioxide and low in chlorine was produced.
U.S. Pat. No. 2,861,871 issued Nov. 25, 1958 to A. Germano, disclosed partial physical separation of produced chlorine from chlorine dioxide by means of selective solubility in the liquid product from the generator.
Other patents, such as the following, mentioned separation of the produced chlorine from produced chlorine dioxide by means of an absorption tower, based on selective solubility in water: U.S. Pat. No. 3,607,027 issued Sep. 21, 1971 to G. O. Westerlund; U.S. Pat. No. 4,137,296 issued Jan. 30, 1979 to D. N. Glew et al; U.S. Pat. No. 4,393,035 issued Jul. 12, 1983 to M. C. J. Fredette; U.S. Pat. No. 4,393,036 issued Jul. 12, 1983 to M. C. J. Fredette; and U.S. Pat. No. 4,543,243 issued Sep. 24, 1985 to H. Frohler et al.
In particular, U.S. Pat. No. 2,871,097 issued Jan. 27, 1959 to William H. Rapson, disclosed the purification of a gaseous mixture of chlorine dioxide and chlorine by treatment with an aqueous solution of sodium chlorite and sodium chlorate. The chlorine reacted with the sodium chlorite to produce more chlorine dioxide, and sodium chloride. However, sodium chlorite is very expensive, and is normally produced from chlorine dioxide.
Yet other patents have been proposed to solve the problem of providing chlorine dioxide solutions having virtually no chlorine by a post-treatment, including the following:
Canadian Patent No. 994,525 issued Aug. 10, 1976 to G. I. Upatnieks et al, which taught the concept of post-treatment involving countercurrent contact of the gaseous mixture of chlorine dioxide and chlorine with an aqueous solution of chlorine dioxide and chlorine which was saturated with respect to chlorine;
Canadian Patent No. 1,118,580 issued Feb. 23, 1982 to D. J. Jaszka et al, which taught the concept of post-treatment involving the use of a mixture of sodium chlorate, sodium chloride and sodium hydroxide at a pH of 4 to 9;
U.S. Pat. No. 2,036,311 issued Apr. 7, 1936 to James F. White, which taught the concept of post-treatment involving the use of aqueous alkali;
U.S. Pat. No. 2,036,375 issued Apr. 7, 1936 to George P. Vincent, which taught the concept of post-treatment involving the use of substantially dry alkali;
U.S. Pat. No. 2,078,045 issued Apr. 20, 1937 to George P. Vincent, which taught the concept of post-treatment involving selectively absorbing gaseous chlorine on lime;
U.S. Pat. No. 2,317,443 issued Apr. 27, 1943 to George L. Cunningham, which taught the concept of post-treatment involving the reaction of chlorine with sulfur and water;
U.S. Pat. No. 2,481,241 issued Sep. 6, 1949 to William H. Rapson et al, which taught the concept of post-treatment involving the addition of gaseous sulfur dioxide; and
U.S. Pat. No. 4,216,195 issued Aug. 5, 1980 to D. J. Jaszka et al, which taught the concept of post-treatment involving the use of an aqueous solution of sodium hydroxide as a scrubbing agent.
Specifically, U.S. Pat. No. 2,833,624 issued May 6, 1958 to J. W. Sprauer, provided a procedure involving continuously feeding, in reacting proportions to produce chlorine dioxide, aqueous solutions of a metal chlorate, an inorganic chloride, hydrogen peroxide, and a strong mineral acid into a reaction zone. The solutions were rapidly mixed and the resulting mixture was maintained at a temperature substantially above 70.degree. C. for a time not exceeding 5 minutes. Reacted mixture was continuously removed from the reaction zone at a rate corresponding to the rate of feed of materials thereinto. The reacted mixture was continuously diluted immediately upon removal from the reaction zone to produce continuously and directly dilute chlorine dioxide product solution. The patentee alleged that the reaction was one in which the production of chlorine was suppressed. Therefore even though it taught that hydrogen peroxide can destroy chlorine dioxide, it did not provide a teaching of post-treatment of a mixture of chlorine dioxide/chlorine with hydrogen peroxide to reduce the amount of chlorine therein.
Specifically, also, U.S. Pat. No. 4,421,730 issued Dec. 20, 1983 to I. Isa et al, allegedly provided for the generation of very pure chlorine dioxide very efficiently and safely in a single generator-crystallizer by reducing an alkali metal chlorate with chloride ion in a strong acid in the presence of hydrogen peroxide and a complex catalyst of palladium (II) with chloride ion. That patent taught a reaction whereby it was possible to manufacture highly pure chlorine dioxide efficiently without generating any acid effluent by reducing an alkali metal chlorate with chloride ion in the presence of hydrogen peroxide and the specified complex catalyst in a single generator-crystallizer under a reduced pressure of about 20 to 400 mmHg. This patent thus taught a process in which large amounts of hydrogen peroxide were consumed during the reaction. It did not teach the post treatment of a mixture of chlorine dioxide and chlorine with hydrogen peroxide, to reduce the amount of chlorine therein.